Gas turbines operate at high temperatures, especially where the combustion system discharges into the inlet of the turbine section. Although satisfactory instrumentation has not been developed to reliably measure the gas temperature in the combustor discharge duct or turbine inlet section, these temperatures can be estimated using turbine parameters that can be reliably measured or determined. Thus, by using measurable or determinable turbine parameters, a control system can be employed to schedule one or more combustion fuel supply circuits.
One such parameter that can be determined, but not directly measured, and used to control the combustion fuel supply circuits is the combustion reference temperature (“CRT”). The CRT is related to the gas temperature at the discharge of the combustor. The CRT can be an important control measure for optimum control of the combustion fuel supply circuits. An accurate determination of the CRT can ensure reliable operation of the gas turbine through proper fuel control and can effectively control NOx and CO emissions in the turbine exhaust gas.
Because the CRT can not be reliably measured during normal operation using conventional systems and methods, gas turbine control systems typically estimate the CRT according to an empirical transfer function derived from previously measured steady state data. This transfer function does not account for transient phenomena such as transport delays, sensor dynamics, and most importantly turbine heat soak, which refers to the amount of energy required to bring the turbine to a steady state operational temperature. Therefore, the estimated CRT used for gas turbine control does not accurately reflect the state of the gas turbine system transiently, such as during loading and unloading. During these transient events, the calculated CRT can differ substantially from the true CRT, resulting in unreliable operation and even a shutdown of the gas turbine.
Thus, there is a need to accurately determine the CRT for the purposes of gas turbine control and reliable operation and a need for systems and methods for modifying the performance of a gas turbine.